Open Windows>3D View Properties>Legend. This tab contains many setting related to the Legend.

To change the font size, simply use the Font Size field to increase or decrease the font. The overall Legend size will be adjusted accordingly

To change the title, disable Autotitle, then type in the Title field (or leave it blank if desired)

Additionally you may want to edit which items are included in the Legend and/or sort them. In the list box at the bottom of the window, use the check boxes to include or exclude individual Legend items. Use the All check box to include or exclude all items. When Sorting is set to Manual, Legend items may be rearranged using Move Up/Move Down commands in right click menu or drag and drop